External quantum efficiency (EQE) of a light emitting diode (LED) is dominated by its internal quantum efficiency (IQE) and light extraction efficiency (LEE). Internal quantum efficiency (IQE) is related to the transition of the photons generated from an active layer when electrons inject into LEDs. Light extraction efficiency is the ability that the light from the active layer emits to medium surrounded. With development of epitaxy technology, Internal lighting efficiency can be up to 80%. However, light extraction efficiency is still low. For example, refraction index of GaN materials is about 2.5 at the range of blue wavelength. The air around them has refraction index of 1. Due to total reflection, the light extraction efficiency in the interface is only 10˜12%.
In addition, light emitting diodes having traditional large size luminescent layers (having area greater than 1 mm×1 mm) usually encounter waveguide effect. Photons are limited in the light emitting diode and rebound within the internal materials so that they are absorbed to lower light extraction efficiency. Due to the fact that phosphors are difficult to be evenly spread on side walls of a light emitting diode package for generating white light, only a limited range of phosphors can be excited, such that light beams from the side walls have little contribution to white light generation. Hence, the entire light extraction efficiency is not good.
Furthermore, the appearance of a traditional light emitting diode is a rectangular pillar. It is another factor causing low light extraction efficiency. Because refraction indexes of semiconductor and package materials are far from that of the air, it makes total reflection angle small. Therefore, photons have little chance to escape from the light emitting diodes. They will be reflected back repeatedly until being absorbed.
In order to solve the problems leading to low light extraction efficiency mentioned above, several inventions had provided better solutions. Please refer to FIG. 1. U.S. Pat. No. 6,229,160 has disclosed a method for designing semiconductor light emitting devices such that the side surfaces (surfaces not parallel to the epitaxial layers) are formed at preferred angles relative to vertical (normal to the plane of the light-emitting active layer) to improve light extraction efficiency and increase total light output efficiency. Device designs are chosen to improve efficiency without resorting to excessive active area-yield loss due to shaping. It is the first one to use truncated inverted pyramid structured light emitting diode and provides light extraction efficiency up to 1001 m/W. However, it is not suitable for GaN light emitting diodes. The key issue is that GaN light emitting diodes usually use sapphire (Al2O3) as a substrate. Sapphire is very hard to be shaped by mechanical process. '160 patent is not commercially useful.
In order to overcome the aforementioned problems, U.S. Pat. No. 6,768,136 provides an improved method. '136 patent uses silicon carbide (SiC) or gallium nitride (GaN) as the material for substrate. With change of the shape of the light emitting diodes, light extraction efficiency can be as good as that mentioned in '160 patent while mechanical process can be easily applied to the substrate. However, silicon carbide and gallium nitride are very expensive to be used as the substrate. It is not workable to replace sapphire with such expensive materials. Therefore, a better invention is still desired.
Obviously, from the above description, the substrate of the light emitting diodes remarkably influences the light extraction efficiency. A better designed substrate can provide more light beams to be released from the light emitting diodes and a competitive manufacturing cost. Hence, the present invention is brought up to provide an excellent solution to all the problems. It can also improve light extraction efficiency of a packaged light emitting diode.